1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device for low-frequency oscillation, and more particularly, to the low-frequency oscillator for easily controlling the duty ratio of an oscillator output signal.
2. Background of the Related Art
In general, an oscillator adopting a constant current source can obtain the desired pulse waveform by charging and discharging a capacitor according to the lo current from the constant current source. A low-frequency oscillator which uses. constant-current charging and discharging needs a first constant current source for charging the capacitor and a second constant current source for discharging. A conventional low-frequency oscillator is disclosed in U.S. Pat. No. 4,642,579, which embodies a capacitor and resistor on a semiconductor chip.
FIG. 1 is a schematic diagram showing a general low-frequency oscillator, in which the oscillator includes first constant current source 20 second constant current source 21, a Schmitt trigger 22, and a capacitor C. First constant current source 20, which is connected to the power voltage (Vcc), furnishes its output current to terminals of capacitor C, second constant current source 21 and Schmitt trigger 22. The other terminals of capacitor C and second constant current source 21 are grounded. The output of Schmitt trigger 22 is fed back to the second constant current source 21. Node 23 includes a test terminal for monitoring voltage V.sub.1 (see FIG. 2) with which capacitor C is charged. Node 24 is an output terminal at which the oscillating output voltage V.sub.2 (see FIG. 3) of the Schmitt trigger 22 is detected. In this conventional low-frequency oscillator, first constant current source 20, which is embodied by a PNP transistor, and second constant current source 21, which is embodied by an NPN transistor, are both realized on the same semiconductor chip. A constant current I.sub.1 from the first constant current source 20 charges capacitor C until the voltage across the capacitor reaches a high threshold voltage (V.sub.B) of Schmitt trigger 22, during which time second constant current source 21 is kept in a state of non-conduction. Upon reaching the high threshold voltage, the output of Schmitt trigger 22 goes high, thereby turning on the second constant current source 21 so that current is discharged via second constant current source 21. This discharging action continues until the charge of capacitor C becomes equal to a low threshold voltage (V.sub.A) of Schmitt trigger 22, whereupon the output of Schmitt trigger 22 falls to its low state. Then, second constant current source 21 turns off again and, accordingly, allows capacitor C to be charged again by the first constant current source 20. These steps are successively repeated so that capacitor C is repetitively charged and discharged. The value of the discharged current becomes the difference between the current value I.sub.1 of the first current source and the current value I.sub.2 of the second current source.
FIG. 2 is a waveform diagram showing the voltage at node 23 of the low-frequency oscillator shown in FIG. 1. The horizontal axis (t) represents time and the vertical axis represents the amplitude of charge voltage V.sub.1 in volts. V.sub.B represents the high threshold voltage of Schmitt trigger 22 and V.sub.A its low threshold voltage.
FIG. 3 is a waveform diagram showing the output voltage at output terminal 24 of the Schmitt trigger shown in FIG. 1. The horizontal axis (t) represents time and the vertical axis represents the amplitude of output voltage V.sub.2. A rectangular waveform thus results from the output to the Schmitt trigger illustrated in FIG. 2.
As shown in FIGS. 2 and 3, the processes of charging and discharging are repetitive. Time T.sub.1 represents the charging duration of capacitor C (from low threshold voltage V.sub.A to high threshold voltage V.sub.B) and time T.sub.2 represents the discharging duration of the capacitor (from high threshold voltage V.sub.B to low threshold voltage V.sub.A). The charging duration is determined by calculating ##EQU1## and the discharging duration is determined by calculating ##EQU2## where the charge and discharge times T.sub.1 and T.sub.2 are expressed in seconds.
As shown in Equations 1 and 2, controlling the amperage of constant currents I.sub.1 and I.sub.2 of the first and second constant current sources 20 and 21 play a very important role on determining the periods of T.sub.1 and T.sub.2. However, the current ratio between the two constant current sources is difficult to control and thus, an accurate duty ratio cannot be obtained simply, because each constant current source (20 and 21) is independently composed of a transistor (PNP or NPN). Thus, unpredictable current irregularities occur due to fluctuations in the supply voltage and the minor amperage differences of the two transistors when initially conducting.